A nanoparticle is a particle whose external dimensions lie on the nanoscale. Its behaviour can depend strongly on its size, shape and surface chemistry, even when its overall chemical chemical composition is fixed.
For a sphere of radius , the ratio of surface area to volume isHalving therefore doubles the surface area exposed per unit volume. At fixed total volume, making many smaller particles exposes more material to the surrounding medium. This helps explain why surface reactions and attached molecules can have a large effect on nanoparticles.[ref]
What Is So Special about Nano? by the National Nanotechnology Coordination Office. Scale at which surface behavior plays a larger role; sphere calculation derived here
The size of an individual nanoparticle also differs from the size of an particle aggregate containing many particles. Reports of particle size consequently need to identify what was measured. Gold nanoparticles provide a visible example: their interaction with light can differ markedly from bulk gold.
A particle aggregate is a cluster made from multiple primary particles. Its overall dimensions and internal arrangement differ from the size and shape of the individual nanoparticles. A measurement reporting particle size must consequently identify whether it resolves individual particles, a connected cluster or an effective size inferred from its motion.
Aggregation changes accessible surface area, fluid transport and interaction with light. A compact cluster and an open, branched cluster containing the same number of particles can therefore behave differently. Some measurement conventions reserve aggregate for strongly bonded or fused particles and use agglomerate for more weakly associated particles; experimental reports should state their convention and how sample preparation affects the clusters.
Deposition of clusters produced in a gas-phase aggregation source. Source conditions control the size distribution, and optional mass selection narrows it. Landing energy and substrate interactions can change the structure delivered to a surface.
A nanoparticle formed from several crystallographic domains joined by twin boundaries. Low-energy external facets can compensate the internal boundary and elastic-strain cost; fivefold motifs cannot extend as an unstrained periodic bulk crystal.
A multiply twinned particle containing twenty approximately tetrahedral domains around a common centre. Its idealized exterior has twenty triangular facets; surface-energy reduction competes with strain and twin-boundary costs.
A fivefold-twinned particle with five approximately tetrahedral domains sharing an axis. Five ideal fcc tetrahedral wedge angles leave an angular deficit of about , accommodated by strain and defects.
Formation of particles or films by delivering precursor droplets into a heated environment where solvent evaporation and precursor decomposition occur. Drying and reaction within droplets can produce dense or porous particles.
A nanoscale particle containing two metallic elements. Spatial arrangements include disordered alloys, chemically ordered alloys, core-shell structures and segregated domains; composition alone does not specify the arrangement.
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